Electrically switchable mirrors and optical components made from liquid-crystal gels

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In liquid-crystal (LC) display devices, patterned electrodes are used to effect switching of molecular orientation within a pixel element, and thin layers of a material (typically a polymer) at the surfaces of the cell plates induce the liquid-crystal molecules to revert to their original orientation after the electric field is switched off [1]. Because of their periodic variation in refractive index, cholesteric LC phases (which have a helical variation in orientation) reflect light at a wavelength determined by the helical pitch [2,3], and so can potentially be used in switchable optical devices such as shutters, reflectors and notch- and band-pass filters. But orientation layers are unable to restore the initial orientation in cholesteric phases. They can instead be given an internal 'memory' of their initial orientation by creating an anisotropic polymer network within the system using in situ photopolymerization [4]. Here we show that such crosslinked cholesteric gels can be used to produce fast electrically switchable reflectors with narrow- to broad-band widths (the latter having a silvery mirror-like appearance). By photopolymerizing in a patterned manner, we can make image recordings in the gels which become visible on application of an electric field. These patterned gels offer the prospect of making optical components such as lenses and gratings by holographic recording.

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